1. Field of the Invention
The present invention relates to a power amplifier having a multi-finger FET and a transmission and reception system.
2. Related Art
In general, as the FET included in the power amplifier, a multi-finger FET which facilitates the current synthesis is widely used. When normal operation is conducted and a gate voltage VG is made constant, the multi-finger FET has static characteristics in which a linear region where a drain current Id is proportional to a drain voltage Vdd and a saturation region where the drain current Id is saturated and the drain current Id becomes nearly constant appear. When heat is generated in the FET, however, a negative resistance component appears in a region which should become the saturation region of the static characteristics and an output current remarkably reduces. As compared with power characteristics in the normal operation, therefore, degraded power characteristics are exhibited.
If in a power amplifier using a plurality of multi-finger FETs heat is generated by each of the multi-finger FETs, the possibility that current phase differences between FETs will appear due to performance degradation caused by the negative resistance component resulting in occurrence of a loop current becomes high. If the loop current begins to occur, a kink phenomenon occurs in the power characteristics and the linearity of the output power to the input power degrades remarkably.
As means for preventing the degradation in linearity, heat is radiated via a substrate by making the gate pitch of the multi-finger FETs large and providing a redundant region in order to mitigate the temperature rise in FET channels. In this case, however, there is a problem that the area of the power amplifier increases.
According to a known technique, temperature balances among a plurality of power FETs connected in parallel are compensated by providing temperature sensors respectively for the power FETs, detecting an output difference among these temperature sensors, and performing negative feedback of the difference output to a gate of one of the FETs (see, for example, JP-A 2003-152513 (KOKAI)).
According to the technique described in JP-A 2003-152513 (KOKAI), however, the temperature sensors are provided on modularized casings, and consequently there is a possibility that a delay time will be caused in detection of temperature changes in FET channels and control will not be able to be exercised. Furthermore, since the temperature changes of the FETs are amplified by using operational amplifiers, a region where the operational amplifiers are provided and new power for driving the operational amplifiers become necessary.
Therefore, it becomes unsuitable to apply the technique described in JP-A 2003-152513 (KOKAI) to the power amplifier including multi-finger FETs.